The use of a magnetostrictive transducer for an ultrasonic dental device, such as a dental scaler, is well-known and standardized throughout the dental profession. Such devices are characterized by having a handpiece into which a removable insert with a working tip is placed. The handpiece contains an excitation coil which is electrically connected via a cable to a control unit, or controller, that provides the excitation energy to the coil. The removable insert contains a stack of plates of magnetostrictive material which expands and contracts when subjected to a time-varying magnetic field. A suitable time-varying magnetic field is created by directing a time-varying electrical current through the excitation coil surrounding the inserted tip, and thereby vibrations are induced in the insert and carried to the tip. The vibrating tip is then used by the practitioner in dental work, a non-limiting example of which is to remove calculus from the surface of teeth.
Although the fundamental concept as described above is widely employed in the same basic form, there is considerable variation in the manner by which the excitation current is controlled, in particular the frequency of the time-varying excitation current. The removable insert has a resonant frequency related to the natural acoustic modes of vibration of the magnetostrictive stack contained therein, and it is desirable to excite vibrations within the insert at or near the resonant frequency. Doing so will optimize the vibrational energy in the insert, and will thus optimize the magnitude of the tip vibration for most efficient use in cleaning the teeth.
There are in practice two common sizes of insert, having resonant frequencies of approximately 25 KHz and 30 KHz, respectively. It is thus desirable that the controller be able to generate time-varying currents at or near each of these frequencies. A number of different device configurations have been developed to accommodate this requirement.
U.S. Pat. No. 5,151,085 to Sakurai, et al. (herein denoted as “Sakurai”) discloses an oscillator for driving an ultrasonic transducer, wherein the oscillator is controlled by feedback from a multi-winding transformer. The transducer of Sakurai, however, is not of the magnetostrictive variety, and does not feature an excitation coil. Instead, Sakurai relies on a rather complex arrangement of inductors, transformers, and amplifiers to detect and match the impedance of the transformer. The handpiece disclosed in Sakurai has no excitation coil; moreover, the controller disclosed in Sakurai is not compatible with magnetostrictive inserts.
Likewise, U.S. Pat. No. 5,180,363 to Idemoto, et al. (herein denoted as “Idemoto”) discloses a complex system built around an oscillator featuring impedance-matching transformers and a phase-locked loop for detecting phase mismatch in the feedback signal. As with Sakurai, Idemoto's handpiece lacks an excitation coil; the transducer disclosed in Idemoto is not of the magnetostrictive variety; moreover, Idemoto's controller is incompatible with magnetostrictive inserts.
U.S. Pat. No. 5,451,161 to Sharp (herein denoted as “Sharp '161”) discloses a magnetostrictive insert with an excitation coil and a transformer for providing feedback to a transistor oscillator. In the oscillator of Sharp '161, the transistor collector-emitter current flows through the primary winding of the transformer, and also through the excitation coil, which is in series with the transformer's primary. The current induced in the secondary winding of the transformer flows into the base of the transistor, thereby causing the oscillator to oscillate near the resonant frequency of the magnetostrictive insert. The oscillator frequency, however, is not precisely at the resonance point of the insert, because there are additional components involved in the feedback circuit which have energy storage effects. Thus, the oscillator frequency is the resonant frequency of the entire circuit, not that of just the magnetostrictive insert itself. Furthermore, the oscillator of Sharp '161 has a limited range of operation, and normally can accommodate only inserts having a restricted range of resonant frequencies. Therefore, to allow the controller to be utilized with inserts having a resonant frequency of 25 KHz as well as inserts having a resonant frequency of 30 KHz, Sharp '161 provides a switchable capacitance in the transformer's secondary circuit, to provide the oscillator with two frequency ranges. Thus, Sharp '161 requires the practitioner to change the switch setting when changing from one type of insert to the other.
U.S. Pat. No. 5,730,594, also to Sharp (herein denoted as “Sharp '594”), partially overcomes the limitations of Sharp '161 by providing a phase-locked loop oscillator to provide automatic tuning. The transformer feedback of Sharp '161 is not suitable for such an arrangement. In addition, Sharp '594 mentions prior art use of a second coil in the handpiece, adjacent to the excitation coil. The second coil provides the feedback for automatic tuning. Besides the need for an additional coil in the handpiece, Sharp '594 also exhibits some limitations in the automatic tuning of the excitation frequency, and therefore provides manual tuning capabilities to overcome those limitations. It is noted that U.S. Pat. No. 6,190,167, also to Sharp (herein denoted as “Sharp '167”), is a continuation of Sharp '594 and presents no additional material.
U.S. Pat. No. 6,241,520 to Gofman, et al. (herein denoted as “Gofman”), discloses a variation on an oscillator which includes the excitation coil as an integral part of the oscillation circuitry. The inductance of the excitation coil substantially determines the frequency of oscillation of the oscillator. Gofman also features ancillary coils and capacitors (“tank circuits”) in the oscillator circuit, so that there are other factors determining the frequency of the oscillation. Thus, as with Sharp '161, as discussed previously, the frequency of oscillation is near, but not exactly at, the resonant frequency of the magnetostrictive insert. Furthermore, Gofman still requires several coils in addition to the excitation coil, thereby incurring additional circuitry complexity and bulk.
U.S. Pat. No. 6,503,081 to Feine (herein denoted as “Feine”) discloses the use of a microprocessor to set the frequency of oscillation, such that the power delivered to the excitation coil is maximized. Feine asserts that the microprocessor can be programmed to sense the power input to the excitation coils, perhaps with the use of auxiliary circuitry or components. Feine, however, does not describe how such programming is to be accomplished, nor specifically how to construct such auxiliary circuitry, nor what such auxiliary components might be. But Feine does suggest using voltage-current phase difference measurements or power response slope measurements to determine the maximum power transfer point, in order to set the oscillation frequency to the resonant frequency of the magnetostrictive insert. Although Feine thus suggests a means of reaching the resonant frequency, the requirement for additional power-measurement circuitry imposes further requirements and limitations.
U.S. Pat. No. 6,819,027 to Saraf (herein denoted as “Saraf”) discloses a controller for driving current into a piezoelectric transducer through a transformer at a constant frequency and power level. The frequency is found during frequency scans as that which delivers peak load current. Saraf does not discuss how his controller can be used to drive a magnetostrictive device, nor does he consider the complex resonance conditions that characterize such a device, one consequence of which is that peak load current does not necessarily occur at the frequency of peak power transfer; the latter is the more desirable operating point.
Also U.S. Pat. No. 4,525,790 to Nakamura discloses a controller for driving current into an ultrasonic device, wherein the frequency is determined as that which achieves peak load current. This, again, does not ensure peak power transfer.
U.S. Pat. No. 5,431,664 to Ureche et al. (herein denoted as “Ureche”) discloses a controller for driving current into an ultrasonic transducer through a transformer while measuring the admittance of the transducer circuit. Ureche advocates operating at a frequency intermediate between series resonance and parallel resonance, attempting to minimize the reactive component of the admittance; he does not, however, teach how to achieve this in a practical and efficient manner, nor does he prove that this would result in maximum power transfer.
U.S. Pat. No. 5,406,503 to Williams, Jr. et al. (herein denoted as “Williams”) discloses a controller for driving current into an ultrasonic transducer through a transformer while directly monitoring the power transferred to the transducer. Drive frequency is determined and maintained so as to deliver maximum power. The disadvantage of Williams' controller lies in the elaborate, and thus expensive, arrangement for continuously measuring the power. There is thus a widely recognized need for, and it would be highly advantageous to have, a means of automatically adjusting the oscillation frequency of the excitation current of a magnetostrictive insert to be substantially at the resonant frequency thereof, in a simple and direct manner that does not require feedback coils, tank circuits, or complex circuitry. This goal is achieved by the present invention.